Ceramic-ceramic composites are particularly useful in high temperature, chemically corrosive environments. This is because such composites are able to withstand chemical corrosion better than comparable metal parts, and because they are better able to withstand thermal and mechanical shock than monolithic ceramic parts.
Ceramic-ceramic composites are described in U.S. Pat. Nos. 4,275,095 and 4,397,901, which describe the method of making a composite article with a carbon fibrous substrate, a pyrolytic carbon sheath, and a compliant coating of metallic carbide, oxide, or nitride over the coated fibers. The fibers are taught to be cellulose, pitch, rayon, polyacrylonitrile, or wool.
U.S. Pat. No. 4,766,013 describes a fibrous substrate with a pyrolytic carbon coating deposited around each fiber, and a metal carbide, boride, oxide, or nitride coating over each fiber. The fibers are taught to be carbonaceous or ceramic.
U.S. Pat. No. 4,863,798 discloses multiple layers of coatings on a carbon or ceramic base. A substrate of chopped fibers, felt, cloth or granular material (graphite or silicon carbide) is mixed with a carbonaceous binder and coated with a pyrolytic carbon layer. A barrier layer of boron nitride or boron carbide is then applied and silicon carbide is coated over that.
A commercially available ceramic composite is marketed under the trade designation "SICONEX" by the Minnesota Mining and Manufacturing Co. (3M) of St. Paul, Minn. SICONE.TM. composites are ceramic-ceramic composites comprised of aluminoborosilicate ceramic fibers, a carbonaceous layer, and a silicon carbide overlayer. SICONEX.TM. composites are prepared by braiding, weaving, or filament winding aluminoborosilicate ceramic fibers in a desired shape, or alternatively, fashioning aluminoborosilicate ceramic cloth into such a shape. The ceramic fiber shape is then treated with a phenolic resin which is cured, producing a rigidified article. The thus rigidified article is heated in an evacuated chamber such that the cured phenolic resin is carbonized. The article is then coated with silicon carbide via chemical vapor deposition at temperatures ranging from about 900.degree. to about 1200.degree. C. to provide a semi-permeable, chemically resistant coating of silicon carbide. The resultant rigid ceramic composite is useful at high temperatures in corrosive environments and has excellent thermal shock resistance. Such materials, appropriately shaped, are commonly used as radiant burner tubes or burner inserts.
One disadvantage to such constructions is that they are permeable to gases. It has been difficult to produce by any of the aforementioned techniques a shaped composite article which is impermeable to gases through the article wall. For some applications, it would be very desirable to have an entirely impermeable construction.
For example, a "thermowell" is a term used in this art for a holder of a thermocouple in a high temperature furnace. Frequently, the furnace atmosphere is chemically corrosive, and thus it becomes important that the thermocouple holder be resistant to chemical degradation.
Conventional ceramic-ceramic composites have been used as thermocouple shields or thermowells, but generally are porous to the atmosphere in the furnace. This can lead to a rapid, undesirable degradation of the thermocouple. It is also difficult to shape long, narrow diameter tubes of ceramic fabric or fiber into long, straight, uniform diameter tubes and to maintain the straightness and uniformity after undergoing the silicon carbide coating process.